Immunoglobulin Light Chain Gene Rearrangements in Childhood Precursor-B-ALL: Immunobiological Characteristics and Applicability for the Detection of Minimal Residual Disease.

Quantitative detection of minimal residual disease (MRD) predicts clinical outcome in childhood acute lymphoblastic leukemia (ALL). It is now generally accepted that MRD-PCR studies should preferably employ at least two immunoglobulin (Ig) and/or T-cell receptor targets per patient. When a high sensitivity of ≤ 10⁻⁴ is included as an extra criterion, the frequency of at least one sensitive PCR target drops to 85 to 90% and the frequency of at least two sensitive PCR targets drops to approximately 80%. Therefore, we aimed to analyze the characteristics of Ig light chain rearrangements as potential additional targets for MRD detection in childhood precursor-B-ALL.

We analyzed Vκ-Jκ and Vλ-Jλ rearrangements in 100 precursor-B-ALL patients using the BIOMED-2 multiplex primer sets in order to evaluate their frequency, characteristics, and applicability as targets for detection of MRD.

Vκ-Jκ rearrangements were detected in 31% of patients. They were significantly more frequent in common-ALL (40%) than in pre-B-ALL (13%) and showed a significant relation with age at diagnosis (highest frequency (55%) between 6 and 10 years) and the presence of TEL-AML1 transcripts (55%). Vλ-Jλ rearrangements were detected in 17% of patients and showed no significant relation with immunophenotype, age or the presence of TEL-AML1 transcripts.

Patients with Vκ-Jκ rearrangements showed a significantly higher frequency of intron-Kde, TCRG, Vδ2-Jα, and TCRB rearrangements than Vκ-Jκ-negative patients. In contrast, incomplete IGH rearrangements were virtually absent. Overall, the mean number of PCR-detectable Ig/TCR gene rearrangements in Vκ-Jκ-positive patients was 4.4. Patients with Vλ-Jλ rearrangements showed a significantly higher frequency of Vδ2-Jα and complete TCRB rearrangements than Vλ-Jλ-negative patients, whereas incomplete IGH rearrangements were completely absent. Overall, the mean number of PCR-detectable Ig/TCR gene rearrangements in Vλ-Jλ-positive patients was 6.4. Of note, in 4 out of 17 patients with Vλ-Jλ rearrangements, no IGK-Kde rearrangements were detected, suggesting that the hierarchy of IGK and IGL rearrangements is less strict then in normal B-cell development.

Vκ-Jκ and Vλ-Jλ rearrangements showed a good stability between diagnosis and relapse (88% and 77% stable, respectively) and reached good sensitivities in RQ-PCR analysis (10⁻⁴ in 82% and 80%, respectively).

Our data indicate that Vκ-Jκ and Vλ-Jλ rearrangements are differently regulated in precursor-B-ALL. We show that all patients with Vκ-Jκ and Vλ-Jλ rearrangements have at least two other Ig/TCR gene rearrangements. Thus, addition of the multiplex Vκ-Jκ and Vλ-Jλ tubes to the PCR panel used for target identification will not increase the number of patients in which at least one Ig/TCR MRD target can be detected. Nevertheless, Vκ-Jκ and Vλ-Jλ rearrangements are relatively more sensitive in RQ-PCR analysis and show a relative high stability between diagnosis and relapse. Therefore, Ig light chain rearrangements may replace TCRG gene rearrangements and may become preferred targets for RQ-PCR-based MRD analysis in childhood precursor-B-ALL. Since both Vκ-Jκ and Vλ-Jλ rearrangements can be detected by one single multiplex tube, such replacement will also limit the number of tubes needed in the initial screening for Ig/TCR MRD targets.